Zeolite is a naturally occuring rock that is refered to as a molecular sieve due to its amazing ability to remove certain compounds from water and hold them inside its structure.  This ability has resulted in the health industry incorporating zeolite into detox tablets to aid the body in getting rid of toxins.  For fish farmers this is really useful as ammonia is one of the compounds so removed.

Hang a bag of zeolite in your fish tank where the water flows across it and the zeolite takes up the ammonia from the water like a sponge.  Once it is `full' of ammonia the bag is then transfered to a brine solution to discharge the ammonia, like squeezing a sponge; whereafter it is again ready for use to suck up more ammonia.  Zeolite is also used in transport tanks and bags to reduce the accumulation of ammonia during transport than can be very stressful to fish.